Diesel engines produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions standards for diesel engines, whether stationary or mobile (e.g. vehicular diesel engines), are being progressively tightened. There is a need to provide improved catalysts and exhaust systems that are able to meet these standards, which are cost-effective.
For diesel engines, an oxidation catalyst (known as a diesel oxidation catalyst (DOC)) is typically used to treat the exhaust gas produced by such engines. Diesel oxidation catalysts generally catalyse the oxidation of (1) carbon monoxide (CO) to carbon dioxide (CO2), and (2) HCs to carbon dioxide (CO2) and water (H2O). Exhaust gas temperatures for diesel engines, particularly for light-duty diesel vehicles, are relatively low (e.g. about 400° C.) and so one challenge is to develop durable catalyst formulations with low “light-off” temperatures.
The activity of oxidation catalysts, such as DOCs, is often measured in terms of its “light-off” temperature, which is the temperature at which the catalyst starts to perform a particular catalytic reaction or performs that reaction to a certain level. Normally, “light-off” temperatures are given in terms of a specific level of conversion of a reactant, such as conversion of carbon monoxide. Thus, a T50 temperature is often quoted as a “light-off” temperature because it represents the lowest temperature at which a catalyst catalyses the conversion of a reactant at 50% efficiency.
Exhaust systems for diesel engines may include several emissions control devices. Each emissions control device has a specialised function and is responsible for treating one or more classes of pollutant in the exhaust gas. The performance of an upstream emissions control device can affect the performance of a downstream emissions control device. This is because the exhaust gas from the outlet of the upstream emissions control device is passed into the inlet of the downstream emissions control device. The interaction between each emissions control device in the exhaust system is important to the overall efficiency of the system.
Oxidation catalysts, such as diesel oxidation catalysts (DOCs), typically oxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas produced by a diesel engine. Diesel oxidation catalysts can also oxidise some of the nitric oxide (NO) that is present in the exhaust gas to nitrogen dioxide (NO2). Even though nitrogen dioxide (NO2) is itself a pollutant, the conversion of NO into NO2 can be beneficial. The NO2 that is produced can be used to regenerate particulate matter (PM) that has been trapped by, for example, a downstream diesel particulate filter (DPF) or a downstream catalysed soot filter (CSF). Generally, the NO2 generated by the oxidation catalyst increases the ratio of NO2:NO in the exhaust gas from the outlet of the oxidation catalyst compared to the exhaust gas at the inlet. This increased ratio can be advantageous for exhaust systems comprising a downstream selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter (SCRF™) catalyst. The ratio of NO2:NO in the exhaust gas produced directly by a diesel engine may be too low for optimum SCR or SCRF catalyst performance.
Whilst it is generally advantageous to include an oxidation catalyst, such as a DOC, that has good NO2 generation activity in an exhaust system, the use of an oxidation catalyst in this way can be problematic when seeking to obtain optimum performance from a downstream emissions control device (e.g. an SCR or SCRF™ catalyst). The average amount of NO2 that is generated by an oxidation catalyst at a given exhaust gas temperature can vary considerably over its lifetime. This can lead to difficulties in calibrating the dosing of the nitrogenous reductant for performing active SCR.